1. Field of the Invention
The present invention relates to a focusing-position-detection circuit for use in optical instruments such as cameras.
2. Description of the Prior Art
Conventionally, as a focusing position detection circuit system, there is known an equal-defocusing-detection circuit system in which two light receiving elements or two light receiving element arrays are symmetrically disposed with respect to a predetermined focusing plane of an object lens, with a small space therebetween. The light receiving elements or arrays detect the degree of defocusing of an image, and the position where the defocusing detected by the two light receiving elements or by two light receiving element arrays is equal is regarded as the focused position of the lens.
Japanese patent publication No. 43-10265 discloses such an equal-defocusing-detection circuit system in which two light receiving element arrays are employed, and the differences in output between adjacent light receiving elements in each light receiving element array are subjected to (i) a non-linear operation such as conversion of differences to absolute values, (ii) the integration of the absolute values and (iii) the comparison of those integrated values. In this circuit system, however, the defocusing is detected only by the comparison of the differences in output of the adjacent light receiving elements. In other words, only high space frequency components of an image on the surface of the light receiving elements are detected and, therefore, the defocusing detection output is generated only in an extremely narrow defocusing range near the focused position. Therefore, when the degree of defocusing is great, the defocusing position detection output is too small to recognize where the focused position of the object lens is, that is in front of or behind the film position.
Hereafter, for the convenience of explanation, when the focusing position of an object lens with respect to an object is in front of the film position, the position of the object lens is referred to as the "front focusing" position. Likewise, when the focusing position of the object lens is behind the film position, the position of the object lens is referred to as the "back focusing" position. When the focusing position of the object lens is exactly at the film position, the position of the object lens is referred to as "focused" position.
Further, two types of equal-defocusing-detection circuit systems are known. In one type, the defocusing detection output, corresponding to the defocusing degree as shown in FIG. 1, is applied to a lens drive motor. In the other type, out of the front focusing, back focusing and focused positions, one focusing state is recognized by the defocusing detection output, and one signal of those possible signals with different values as shown in FIG. 2 is applied to the lens drive motor. In the former type, since an analog signal is applied to the lens drive motor, the stop operation for stopping the object lens exactly at the focused position tends to be unstable. In the latter type, when the lens is driven from the front-focusing range or from the back-focusing range to the focused range by the lens drive motor, the lens has to be stopped immediately when the lens reaches the focused range. However, over-run from the focusing position of the lens is apt to occur and it is difficult to perform accurate focusing operations. Furthermore, when the defocusing is extreme or when the contrast of the object is low, the contrast of the image of the object is significantly reduced on the light receiving element arrays. The result is a risk that the system cannot distinguish between the front focusing, the back focusing and the focused position.